I. Field of the Invention
The present invention relates to the field of imaging. More precisely, the present invention relates to pixel cells of image sensors.
II. Background of the Invention
Imaging arrays, used to produce an image representing an object, are typically formed of rows and columns (bit lines) of photo detectors (pixel cells). The pixel cells generate photo charges proportional to light reflected from an object to be imaged. Photo charges from each pixel cell are converted to a signal (charge signal) or potential representative of a level of energy reflected from a respective portion of the object. The signal or potential is read and processed by video processing circuitry to create an image representing an object.
Pixel cells belongings to a same bit line are usually connected at a common output node from where a signal or potential, representative of the level of energy, is read out.
Pixel cell cells belonging to the same bit line "see" an overall capacitance (hereinafter referred to as "bit line capacitance"), at the common output node. Each pixel cell in a same bit line is individually controlled to read out at the common output node. Typically, pixel cell cells belonging to a same row are commonly controlled by a same signal (word line) such that an entire row may be read out at a substantially same time.
To meet the increasing need for high speed image sensor devices, it has become necessary to integrate image sensor arrays with digital circuitry that controls the operation of the array and processes the array's output. Integration of image sensors with complementary-metal-oxide-semiconductor (CMOS) support circuitry is most desirable because of the low power consumption characteristics and common availability of CMOS technology. Such an imaging array integrated with CMOS support circuitry is called CMOS active pixel cell sensor (GAPS) array.
Typically, a pixel cell includes a photosensor that detects light impinging thereon and "converts" the light into an electronic signal indicative of an intensity of light detected by the pixel cell cell. A driving device receives the electronic signal and drives a current proportional to the electronic signal to a bit line to which the pixel cell is coupled. Then the pixel cell cells in a selected row are accessed by asserting the WORDLINE signal to each pixel cell access device of each pixel cell of a selected row. Then each bit line to which a corresponding pixel cell of the selected row is couple, may be charged by a current driven by the driving device of the pixel cell to a voltage level representative of an entire row may thus be read out at a substantially same time. The pixel cell cells of other rows, not currently accessed, have their pixel cell access devices switched off by de asserting the word line signals corresponding to these rows.
The industry trend in digital imaging has been towards higher and higher image resolution supported by smaller pixel cells. Smaller pixel cells lead to smaller and cheaper optical systems and cameras. It is desirable to provide an active pixel cell sensor array with reduced size pixel cells.